Structure of bacterial cell:
These are prokaryotic, microscopic
with simple organization of body than eukaryotes. Their cells lack nucleus,
membrane bound organelles, endomembrane systems. Cell wall surrounds the body
as outer membrane and makes them rigid. Capsule is the outermost layer that is
present above the cell wall.
Size:
Bacteria are very small in size. They
usually range from 0.2 to 2 μm in diameter, 0.5 to 5 μm in length. Example: Mycoplasmas (0.3 micrometers in diameter). Few
bacteria are very large in size and are visible to the naked eye. Example: Thiomargarita namibiensis, Epullopiscium fisheloni which is rod
shaped is about 600 micrometers in length and 75 micrometers in diameter.
Due to their small size cell body
their surface area to volume ratio is more i.e., no internal part is far from
the surface and nutrients can easily and quickly reach all the parts of the
cell. It also allows rapid uptake and intracellular distribution of nutrients
and excretion of wastes. At low surface area to volume ratio, the diffusion of
nutrients and waste products across the cell membrane slows down the metabolic rate
which makes the cell less chances to survive.
Shape:
As we previously discussed that the bacteria occur in four basic
structures like cocci (spherical in shape), bacillus (rod shaped), vibrio (comma
shaped), spirillum (spiral shaped) etc. Some are pleomorphic (Corynebacterium). The term bacillus has
two meanings. It refers to a bacterial shape as well as genus. For Example: Bacillus anthracis, the causative agent
of anthrax, Bacillus aerius, Bacillus
subtilis etc.
Cell wall:
Usually most of the bacterial cells
are bounded by a complex cell wall which is made of peptidoglycon. It is unique
characteristic in bacterial cells. Peptidoglycon is also termed as murein or
mucopeptide.
Peptidoglycon
is a polymer containing two sugar derivatives N-Acetyl glucosamine (NAG) and
N-Acetylmuramic acid (NAM). These are joined through β-1,4 glycosidic bond. NAM is similar to NAG with the exception
of presence of lactic acid connected by ether linkage. The carboxylic group in
NAM is attached by a tetra peptide which is a peptide chain of four alternating
D and L amino acids like L-Alanine, D-Alanine D-Glutamic acid, and either
L-Lysine or meso diamino-pimetic acid (DAP).
Few
bacteria lack cell wall or may present little amount. Example: Mycoplasmas,
L-form bacteria or L-Phase bacteria. L- Forms can be generated cell wall in the
laboratory from species like Bacillus subtilis and Escherichia coli which have
cell wall by by inhibiting peptidoglycon synthesis or by treating with
lysozyme. But unlike L forms Mycoplasmas cannot derive from the bacteria having
cell wall.
Bacteria can be identified by using
staining techniques which uses basic dyes such as crystal violet, methylene
blue. Based on the results the type of bacteria can be identified. Example:
Acid fast staining technique used for identification of Mycobacterium species,
Gram staining technique is used for the differentiation of type of bacteria
based on their cell wall composition, flagella staining techniques etc.
Gram Positive bacteria:
The bacteria that give positive
result during staining technique are Gram Positive Bacteria. They take up the crystal
violet color after the decolorization of rest of the sample due to presence of 20
to 80nm thick peptidoglycon layer outside the plasma membrane. The thinner or
smaller periplasmic space is present between plasma membrane and cell wall. These
bacteria have many teichoic acids attached covalently to the NAM of
peptidoglycon layer or to the lipids of the plasma membrane. The teichoic acids
are polyol phosphate polymers (polymer of ribitol or glycerol joined by
phosphate) bearing strong negative charge. Mesosomes are more prominent.
Gram Negative bacteria:
The bacterium that doesn’t retain the
color of crystal violet after the decolorization step in staining technique is Gram
Negative bacteria. This is due to absence or presence of small amount of
peptidoglycon in the cell wall. These have more periplasmic space between cell wall
and plasma membrane. They lack teichoic acids. Outside the cell wall layer
similar to plasma membrane outer membrane is present. The constituents in outer
membrane are not completely similar to plasma membrane. It contains lipopolysaccharides,
lipoproteins, proteins and phospholipids. The lipids and polysaccharides are
linked in the outer membrane called lipopolysaccharides. So it is named as
lipopolysaccharide layer or LPS layer. LPS consists of three components, the
core polysaccharide, O-polysaccharide, Lipid A. Lipid A is highly toxic, and as
a result the LPS act as an endotoxin. The O-Polysaccharide functions as an
antigen and is used for distinguishing Gram Negative bacteria.
Glycocalyx:
It is a thick, high molecular weight
secretary substance present in many bacteria. It is present outside to the cell
wall composed of polysaccharide, polypeptide or both. The Glycocalyx may be
thick or thin, rigid or flexible depending on their chemical nature. The terms Capsule
and Slime layer are frequently used to describe Glycocalyx layers.
The capsule is rigid layer organized in a tight matrix having polysaccharides.
Presence of Capsule makes bacteria resistant to phagocytosis and enhances their
virulence. The slime layer is loosely deformed and loosely attached to the cell
wall.
Plasma membrane:
It is present below the cell wall and also
called as cell membrane. It is common in both prokaryotes and eukaryotes. It is
composed of phospholipid bilayer. The only difference between them in the
composition of plasma membrane is the presence of sterols and hopanoids in the
eukaryotes and prokaryotes (except Mycoplasmas; contains sterols) respectively.
Plasma membrane performs functions such as transport, energy transduction. The
invasions in plasma membrane are Mesosomes which play role in formation of cell
wall during cell division, chromosome replication. These are present both in
Gram Positive and Gram Negative Bacteria.
Cytoplasm:
It
is the inner content of the cell which is prior to the plasma membrane. It
contains 80% of water, 70s ribosomes, chromosomal material with nuclear
envelope, Circular DNA plasmids, proteins, carbohydrates, lipids, inorganic
ions, many low molecular weight compounds. It lacks unit membrane bound
organelles.
The
sub units of 70s ribosomes in prokaryotes are smaller subunit 30S containing
one molecule of rRNA (16S rRNA) and larger subunit 50S containing of two
molecules of rRNA (23S rRNA and 5S rRNA). The cytoplasm also contains inorganic
and organic substrates called inclusion bodies. They are membrane bound or
membrane less. Their unit membrane varies in the composition. Some may be
proteinaceous or some may be with lipids.
Inclusion bodies:
1. Glycogen: It is a branched polymer of glucose
and present in the form of granules in the cytosol. It acts as a storage
unit for reserve food materials. They survive more than glycogen less bacteria.
Glycogens with iodine appear in reddish brown color.
2. Carboxysomes: These are polyhedral micro
compartments that fix CO2 by the enzyme RuBisCo (Ribulose-1,
5-Bisphosphate carboxylase) in Cyanobacteria. These are covered by
proteinaceous shells. There are two types of Carboxysomes: α Carboxysomes and β
Carboxysomes which appear similarly but differ in their protein composition.
3. Polyphosphate granules: Also called volutin granules. These
are reserve form of inorganic phosphate present in the cytoplasm of some
bacteria. They show red or a different shade of blue when stained with blue
basic dyes, methylene blue or toluidine blue. This is known as Metachromatic
effect. Hence these granules are also called as Metachromatic granules. These
are synthesized for both energy and phosphate storage.
4. Cyanophycin granules: it is present in cyanobacteria and
heterotrophic bacteria. it is non ribosomally produced amino acid polymer.
5. Magnetosomes: these are membrane bound inclusion
bodies that show movement according to magnetic field. These are tiny chain
structures surrounded by lipid bilayer. Magnetosomes are present in
magnetotactic bacteria. The movement that magnetotactic bacteria show is called
magnetotaxis. These are motile and mostly aquatic. They usually contain
crystals of magnetite (Fe3O3) or crystals of greigite (Fe3S4).
It acts like a compass needle to orient magnetotactic bacteria in geomagnetic
fields.
6. Gas vesicle: These are hallow spindle shaped
structure made of protein called GvpA and GvpC but absent in eukaryotes. These
proteins made bacteria to float on the surface of water. This makes
phototrophic bacteria able to trap light. It is impermeable to liquid water,
but it is highly permeable to gases. It provides buoyancy to planktonic cells
by decreasing their overall cell density.
Surface appendages: Flagella, pili and fimbriae. These are the external
appendages lies on the surface of the body. Each appendage has different
function.
Flagella:
These help bacteria for
motility. They differ in size in different bacteria. It is usually present in both Gram positive
and Gram negative bacteria. It has mainly three parts filament, hook and basal
body.
Filament: It lies external to the cell
surface. The filament is made of flagellin protein. As shown in the figure the
flagellin proteins are assembled to form a cylindrical structure with hallow
core. It is a rigid structure.
Hook: It is highly curved structure present
at the end of the filament as shown in the figure.
Basal body: the basal body is the part to which
the hook is attached and makes flagella to propel. It has four rings MS ring
(Membranous, Supra membranous), P ring (Peptidoglycon), L ring (Lipopolysaccharide),
and C ring (Cytoplasm). The C ring has three proteins FliG, FliM and FliN which
makes flagella to rotate. It interacts with the motor proteins. The C ring is
also called as Switch complex as it can switch the direction of the flagellar
motor. Based on the number and arrangement of flagella on the surface of
bacteria they are of 4 types. They are
·Artichous:
Bacterium without flagella.
·Monotrichous: Single flagellum on one side of
bacterial surface.
· Amphitrichous:
Single flagellum on both sides of bacterial surface.
· Lophotrichous:
Cluster of flagella on one side of bacterial surface.
·Amphilotrichous:
Cluster of flagella on both sides on the surface of bacteria.
· Peritrichous:
Flagella present all over the body of bacteria.
Pili:
It is also called as Sex pili as it plays an important role
during conjugation. It is a thin, hair like appendage present on the surface of
the bacteria. it is usually present in Gram negative bacteria and rarely in
Gram positive bacteria.
Sex pili
Function of Pili in conjugation
Fimbriae:
These are smaller hair like projections spread all over the
body. It is present mostly in Gram negative bacteria and some in Gram positive
bacteria. It helps the bacteria to adhere to the surface. It is also known as
“attachment pili”.
Endospores:
These
are the heat resistant spores that can withstand harsh conditions such as heat,
UV radiation, γ radiation, chemical disinfectants and desiccation, produced
by some Gram positive bacteria. They are Bacillus, Clostridium and Sporosarcina.
It develops within the cell. Endospores were discovered by Tyndall in his
process called Tyndallization.
The ability to produce spores also is of ecological advantage
to the organism as it enables it to survive under adverse conditions. Thus
formation of spores is generally occurs under conditions of nutrient depletion.
The endospore structure is very complex and has many layers
that are absent from the vegetative cell. Exosporium is a thin delicate protein
covering, Spore coat lies beneath the exosporium composed of several layers of
protein, Cortex is present beneath the spore coat, made up of a peptidoglycon.
Spore cell wall is present beneath the cortex and surrounds the protoplast. It
contains ribosomes and nucleoid which is non functional.
The endospores contain dipicolinic acid complexed with calcium which makes spore heat resistant. Staining method is used for the identification of morphology of spore in different bacteria by using Schaeffer-Fulton endospore stain.
Differences between Gram positive and Gram Negative bacteria:
|
Property
|
Gram negative
bacteria
|
Gram positive
bacteria
|
|
Cell wall
|
2 to 7 nm thick
|
20 to 80 nm thick
|
|
Teichoic acids
|
Absent
|
Present
|
|
Periplasmic space
|
More than Gram positive bacteria
|
Negligible
|
|
Outer membrane
|
Present
|
Absent
|
|
Motility
|
Usually motile
|
Mostly non motile
|
|
Appendages
|
Usually present
|
Rarely present
|
|
Flagellar structure
|
4 rings in basal
body
|
2 rings in basal body
|
|
Endospores
|
Cannot produce Endospores
|
Some genera produce endospores.
|
|
Gram reaction
|
Retains purple color
|
Does not retain
color.
|
Genetic material:
The bacterial genome
is called nucleoid or genophore which contains one chromosome. It lacks nuclear
membrane. It comprises a single circular, covalently closed, double stranded
helical, negatively super coiled molecule of DNA. Bacterial chromosome contains
DNA and proteins. The bacterial chromosome forms domains which made fit into
the nucleoid. This domain organization enables the chromosomal DNA to undergo
structural changes during different cellular processes like replication,
transcription, segregation that occur simultaneously in a bacterial cell. The
nucleoid associated proteins that help in compaction of DNA are HU (Heat
Unstable nucleoid protein), HNS (Histone like Nucleoid Structuring), IHF
(Integration Host Factor), and SMC (Structural Maintenance of Chromosomes).
These proteins also involve in cellular processes. The Archaebacteria contain
histones which is the common feature in both Archaea and Eukarya domains. The
genome in bacteria is circular due to lack of telomeres which are present in
Eukaryotes.
|
BACTERIA
|
CHROMOSOME
(Number, Shape and Size)
|
|
Agrobacterium
tumifaciens
|
One linear (~2.1
Mb) + one circular (~3.0Mb)
|
|
Bacillus subtilis
|
One
circular (~4.2Mb)
|
|
Brucella
melitensis
|
One
circular (~2.1 + ~1.2 Mb)
|
|
Vibrio
cholerae
|
Two
circular (~2.9 + ~1.1 Mb)
|
|
Rhodobacter
sphaeroides
|
Two
circular(~3.0 + ~ 0.3 Mb)
|
|
E.coli
K-12
|
One
circular (~4.6 Mb)
|
Plasmids:
The plasmids are usually circular, negatively supercoiled self
replicating double stranded DNA molecule that are maintained as discrete,
extrachromosomal genetic elements in bacteria. Plasmids are usually smaller than
chromosomes in bacteria. They usually appear varying from less than 5 kbp to
more than several hundred kbp. Some bacteria also contain about 2Mbp plasmids. Linear
plasmids also appear in some genera Borrelia and Streptomyces and Clostridium.
Plasmids are considered as replicons, units of DNA capable of replicating autonomously
within a suitable host. They contribute to about 0.5 to 5 % of total DNA.
Joshua Lederberg
